In a marine seismic streamer, which may have a length of 2-3 km, as a rule a plurality of hydrophones are spaced from one another rather uniformly along the length of the streamer. The hydrophones are designed as pressure cells, the connecting lines of which are usually exposed. Since a streamer is normally filled with oil of very low conductivity, insulation of the connecting elements of the hydrophone is not necessary.
However, such a design of a streamer also has risks because, in the event of a leak of the streamer, seawater can penetrate into the interior thereof and thus neutralize the insulation. In such an event, the defective portion of the streamer cannot be used any longer from the electrical point of view. If the hydrophones and electrical connections are exposed to the seawater for a long time, corrosion occurs which causes the pertinent hydrophones to be useless. A streamer which is defective in such a manner must therefore be extensively repaired and rebuilt.
Hydrophones which are employed in the known streamers comprise essentially pressure cells each of which typically comprises two pot-like half shells which are directed toward one another and which are soldered to one another at the abutting edges. Sensor elements in the form of piezoelectric disks are normally fitted onto the insides of the cell surfaces, forming the pressure membranes. Electrical connections to the membranes are made by adhesive bonding. To compensate for acceleration influences, two sensors are in each instance connected oppositely from the electrical point of view.
In the manufacture of such pressure cells, the two cell halves are soldered to one another at the edge side by soft solder. The heat arising in this case leads to a situation in which no stable pressure level can be achieved within the cell, so that the air pressure prevailing in the interior of the pressure cell in the cooled condition varies greatly from hydrophone to hydrophone. For this reason, a hard soldering of the cell halves does not enter into consideration.
Marine seismic streamers are normally used at water depths of up to 30 m. These hydrophones exhibit a depth limitation which arises because at greater depths the mutually opposite piezoelectric sensors press against spacing bodies and lose their measuring capability as a result. Direct impact of the piezoelectric crystals is impermissible because such direct impact against microcrystalline structures easily destroys the piezoelectric crystals. Furthermore, edge-side releasing of the adhesive bond between membrane and piezoelectric sensor can take place, so that the rejection rate of the hydrophones increases exponentially upon sinking below a predetermined depth.
FR 2,122,675 discloses a hydrophone which has two membranes which are disposed parallel and spaced from one another. The spacing of the membranes from one another is established by annular reinforcements on the outer periphery. The membranes may be made of metal and have piezoelectric pressure transducer elements peripherally mounted.
DE 3,732,401 discloses a piezoelectric hydrophone which likewise exhibits transducer elements which are mounted on opposite membranes. In this case, the transducer elements are disposed on the inside of the membranes, which consist of stainless steel, and which are welded to the lateral parts by laser beam welding. In the interior of the hydrophone and on the outsides of the membranes, there are disposed mechanical overload protection elements, which limit the membrane deformation.
DE-OS 3,931,578 discloses a piezoelectric membrane hydrophone in which the piezoelectrically activatable foil is clamped into an insulating frame and is provided, on opposite surfaces, with electrodes which overlap one another in a partial region. On that side which is remote from the measurement object, the foil is covered over by an electrically insulating sealing compound layer, which also entirely surrounds the clamping frame.
It is also known to fit the piezoelectric sensors on the outside of the membranes and to coat them with a protective lacquer. In this case, the membranes are welded to one another at the edge sides. Such a hydrophone, which was disclosed in a paper presented at the 51st EAEG meeting by the company Prakla-Seismos, has not however to date entered into use, since the fundamental problems of the aforementioned nature could not be adequately controlled even with this type.